Fibrillar product of electrostatically spun organic material

ABSTRACT

A product comprising a mat of fibers prepared by electrostatically spinning an organic material and collecting the spun fibers on a suitable receiver.

This invention relates to the production of fibrillar products by theelectrostatic spinning of organic materials.

The technique of electrostatic spinning of liquids, including solutionscontaining a fibre-forming material, is known and has been described ina number of patents as well as in the general literature.

The process of electrostatic spinning involves the introduction of aliquid into an electric field, whereby the liquid is caused to producefibres which tend to be drawn to an electrode. While being drawn fromthe liquid the fibres usually harden, which may involve mere cooling(where the liquid is normally solid at room temperature, for example),chemical hardening (for example by treatment with a hardening vapour) orevaporation of solvent (for example by dehydration). The product fibresmay be collected on a suitably located receiver and subsequentlystripped from it.

The fibres obtained by the electrostatic spinning process are thin, ofthe order of 0.1 to 25 micron, preferably 0.5 to 10 micron and morepreferably 1.0 to 5 micron in diameter.

We have found that the fibres, if collected to form a mat of appropriatethickness may, because of the inherent porosity of the mat so obtained,provide a non-woven material having a wide variety of applications,depending upon the composition of the fibres, their density ofdeposition, their diameter, and their inherent strength, and thethickness and shape of the mat. It is also possible to post-treat suchmats with other materials to modify their properties, for example toincrease their strength or water resistance.

Fibres having different properties may be obtained by adjusting theircomposition either by spinning a liquid containing a plurality ofcomponents, each of which may contribute a desired characteristic to thefinished product, or by simultaneously spinning from different liquidsources fibres of different composition which are simultaneouslydeposited to form a mat having an intimately intermingled mass of fibresof different material. A further alternative is to produce a mat havinga plurality of layers of different fibres (or fibres of the samematerial but with different characteristics e.g. diameter) deposited,say, by varying with time the fibres being deposited upon the receivingsurface. One way of effecting such a variation, for example, would be tohave a moving receiver passing in succession sets of spinnerets fromwhich fibres are being electrostatically spun, said fibres beingdeposited in succession as the receiver reaches an appropriate locationrelative to the spinnerets.

Thus, the present invention provides a mat comprising a plurality offibres of organic material, said fibres being obtained by electrostaticspinning from a liquid comprising the material or a precursor thereof.

Within the term mat we include deposits of electrostatically spun fibresin the form of three dimensional as well as two dimensional articles.

The invention will be further understood from the following moredetailed description taken with the drawings in which:

FIG. 1 is a schematic perspective view of an apparatus forelectrostatically spinning and collecting fibres;

FIG. 2 is a schematic perspective view of a second embodiment of aspinning and collecting apparatus;

FIG. 3 is a schematic perspective view of a third embodiment of aspinning and collecting apparatus; and

FIG. 4 is a perspective view of a wound dressing.

According to one embodiment of the present invention we provide a shapedmat of electrostatically spun fibres in a form appropriate for use as awound dressing. A particular advantage of the use of materials made fromthe electrostatically spun fibres is that the fibres may be of verysmall diameter, to give a mat with small interstices and consequently ahigh surface area. Where the dressing is formed from a wettable polymer,blood or serum escaping from the wound tends to penetrate the dressingand the high surface area encourages clotting. Such dressings may beused as emergency dressings to halt bleeding. As examples of suitablepolymers we may mention polyurethanes. Where the dressing is formed froma non-wetting polymer a particular advantage is that if the intersticesbetween the fibres are sufficiently small, averaging, say 1 to 100 μ,tissue fluids, including blood, tend not to permeate the dressing, sothat the fluids are retained adjacent to the wound, where clotting willoccur. Subsequent removal of such a dressing is facilitated by theabsence of blood clot permeating the dressing material. Furthermore suchdressings have the advantage that they are usually sufficiently porousto allow interchange of oxygen and water vapour between the atmosphereand the surface of the wound. As examples of suitable non-wettingpolymers we may mention saturated polyesters e.g. polyethyleneterephthalate, fluorinated compounds, particularly fluorinatedhydrocarbons, e.g. PTFE, and silicones.

Such dressings may, of course, be associated with suitable supports orreinforcement, with mats of, say, woven fibres which may have otherdesirable properties, or with surface or other treatment with materialshaving antiseptic or wound-healing properties. Blood clotting, forexample, may be encouraged by incorporating clotting accelerators orinducers in or on the mat and/or on materials associated therewith in awound dressing. Other components with which the mat may be associatedinclude water-proof layers intended to protect the mat from undesirableeffects of moisture, dirt etc.

Preferably the wound dressing of the invention comprises a mat offlexible non-absorbent, porous, hydrophobic material, together with anon-absorbent backing layer. Such a backing layer is preferably made ofhydrophobic material, but this is not essential. The dressing may alsoinclude means for applying pressure to the mat. Such means may be, forexample, a stretchable elastic bandage.

In a preferred embodiment the dressing comprises a backing layer, onesurface of which has an adhesive facing, and on the same surface of thebacking strip a porous mat of the material of the invention; optionallya pad of absorbent or not-absorbent material is located between thebacking layer and the mat.

Non-adherent dressings according to the invention have been tested forefficacy by applying them to the surface of a wound on a rabbitinvolving loss of an area of full thickness skin, and observing theprogress of healing in comparison with control wounds. A dressing madefrom the preferred material allowed normal healing with little or noseepage of fluid through the dressing and minimal adherence of thedressing to the scab.

According to a further embodiment of the invention we provide a shapedmat comprising electrostatically spun fibres in the form of a lining orsurfacing to a component which may be in contact with body fluids suchas blood and lymph. Such mats may be tubular, or of irregular shape.

The difficulty of development of satisfactory blood and body tissuecompatible surfaces on, say, the walls of artificial hearts and othercirculatory assisting devices, as well as compatible linings to damagednatural as well as artificial blood vessels, represents an obstacle tothe development of safe artificial organs and tissues. We have foundthat the deposition upon the surfaces of such artificial organs andtissues of a lining of thin fibres of appropriate material may improvetheir blood and other tissue fluid compatability. It is desirable forthis purpose, however, that the lining is very thin and the use of anelectrostatically deposited fibrous coating has been shown to meet manyof the critical requirements. The primary desiderata include

a. very small fibre diameters (small in relation to cell dimensions), sothat fibre diameters of 0.1 micron to 10 micron, and particularly 0.5 to5 micron are particularly appropriate.

b. The lining should be sufficiently porous to allow penetration ofcells into it; ideally therefore the average pore dimension should be ofthe order of 5 to 25 micron, preferably 7 to 15 micron.

c. The lining should preferably be of the order of 10 to 50 micron inthickness.

d. The lining should be capable of being bonded to the article to whichit constitutes a lining by some suitable means not involving a processdestructive of the properties indicated above.

e. The lining should contain no materials harmful to the body or to thebody cells or fluids which may come into contact with it.

The technique of electrostatic spinning provides a method of formingsuch linings to accord perfectly with the dimensions and contours of thearticles to be coated by making the surface of the article or positiveor negative replica thereof the collector in an electrostatic spinningprocess.

Materials suitable for the preparation of such linings include polymericsubstances and in particular inert, polymeric substances. As preferredsubstances we would mention fluorinated hydrocarbons, e.g. PTFE whichconveniently may be spun from a dispersion of the material in a suitabledispersing agent, and polyurethanes which may be spun from solution.

In some applications the mat may be strong enough, or may be spun thickenough, to be used without a supporting article i.e. it will notproperly be described as a lining. Thus self-supporting tubular devicesmay be electrostatically spun; for example vascular prosthetics may beprepared from polytetrafluoroethylene or from polyurethanes, etc.

The electrostatically spun products, for example tubes or other shapeditems, may as described above, be of sufficient strength to be employedas such, without reinforcement. However, it is usually preferred thatthe material is reinforced, for example by applying to one surface ofthe product a reinforcing layer, which itself may be electrostaticallyspun, or by incorporating reinforcement within the wall of the productitself. Thus, we have reinforced electrostatically spun products byincorporating within the wall thereof a web which may be woven ornon-woven, or an alternative arrangement of fibres. We particularlyprefer to employ as reinforcement a helix of suitable fibre, said helixbeing located within the walls of a tubular product comprisingelectrostatically spun fibrous material. Although it is usual to enclosethe reinforcement within the wall material we do not exclude thepossibility of applying it to a surface of the product where itspresence will not be disadvantageous. The thickness of reinforcementwill be influenced inter alia by the thickness of the mat the locationof the reinforcement and the reinforcement strength required. In generalthe thickness of the reinforcement will be less than that of the mat,although where the reinforcement lies at a surface of the mat and mayproject therefrom the thickness of the reinforcement may be thicker thanthat of the mat. Generally the thickness of the reinforcement (or ofreinforcing fibres) will be of the order of 0.1 to 10 times thethickness of the mat preferably 0.2 to 0.8 times.

Suitable reinforcing materials include metallic, polymeric or glassfibre. Such electrostatically spun tubes and other prosthetics have theadvantage over tubes used hitherto in this application in generating athinner layer of encapsulating natural tissue so that smaller diametertubes may be used without the tube becoming clogged by natural tissue.

The mats according to the present invention may be spun from a solutionof or a dispersion of a polymer or its precursors. Polymers which may beconveniently spun from solution include high molecular weight fibreforming thermoplastics; in particular we would mention polyurethane,polamides and polyacrylonitrile. Polymers which may conveniently be spunfrom dispersion include polytetrafluoroethylene and polyesters as wellas those listed above. As an example of a polymer precursor which may bespun from solution we mention urea formaldehyde which may becross-linked subsequent to spinning by treatment with acid vapour.

Water soluble polymers, e.g. polyvinyl alcohol, polyvinyl pyrrolidone,and polyethylene oxide, may be spun from aqueous solution. While we donot exclude the possibility that mats prepared from such materials maybe used as prepared, preferably such mats are given at least a degree ofinsolubility in aqueous medium e.g. by cross-linking with a suitablereagent.

Where the mats are spun from a dispersion the spinning materialcomprises preferably also a solution of an additional component whichacts to enhance the viscosity of the suspension and to improve its fibreforming properties. Most convenient for this purpose, we have found, isan additional organic polymeric material which subsequent to fibreformation, can, if desired, be destroyed during sintering.

The preferred spinning material, then, is a solution or suspension whichpreferably comprises an organic polymer in an amount such that it iscapable of forming a fibre and has cohesion properties such that thefibre form is retained during any post fibreization hardening until thefibre has hardened sufficiently not to lose its fibrous shape ondetachment from a support where this is appropriate.

Where mats are spun from solution they comprise point bonded fibres andare often strong enough for use without any further treatment.

Where mats are spun from dispersion they often have a tendency to befriable, being mere agglomerations of discrete particles held togetherin the form of fibres by the additional organic polymeric componentpresent. Preferably such mats are sintered so that the particles softenand flow into each other and the fibres may become point bonded. In thecase of PTFE sintering may conveniently be carried out between 330° and450° C, preferably between 370° and 390° C. Sterilisation may proceedconcurrently during the sintering process. The sintering temperature inthe case of PTFE is usually sufficiently high to destroy completely anyundesirable organic component in the final product e.g. material addedsolely to enhance viscosity or emulsifying agent.

The additional organic component need be employed only in a relativelysmall proportion (usually within the range 0.001 to 12% and preferably0.01 to 3%) by weight of the suspension, although the preciseconcentration for any particular application can easily be determined bytrial.

The degree of polymerisation of the additional organic component ispreferably greater than about 2000 units linearly; a wide range of suchpolymers is available. An important requirement is solubility of thepolymer in the selected solvent or suspending medium which is preferablywater. As examples of water-soluble polymeric compounds we may mentionpolyethylene oxide, polyacrylamide, polyvinyl pyrrolidone and polyvinylalcohol; where an organic medium is employed to prepare the spinningmaterial, either as the sole liquid solvent or as a component thereof, afurther wide range of organic polymeric compounds is available, forexample polystyrene and polymethylmethacrylate.

The degree of polymerisation of the polymer will be selected in thelight of required solubility and the ability of the polymer to impartthe desired properties of cohesion and viscosity to the fibreizableliquid.

We have found that generally the viscosity of the fibreizable liquidwhether due solely to the presence of the fibreizable polymer or partlycontributed to by the additional organic polymer should be greater than0.1 but not greater than 150 poise. Preferably it is between 0.5 to 50poise and more preferably between 1 and 10 poise, (viscosities beingmeasured at low shear rates). The viscosity required using a givenadditional organic polymer will vary with the molecular weight of thepolymer, i.e. the lower the molecular weight the higher the finalviscosity needed. Again, as the molecular weight of the polymer isincreased a lower concentration of it is required to give goodfibreization. Thus, as examples we would mention that in the preparationof polytetrafluoroethylene mats we have found that using a polyethyleneoxide of MW 100,000 as the additional organic polymer a concentration ofabout 12% by weight relative to the PTFE content is needed to givesatisfactory fibreization, whereas with a MW of 300,000 a concentrationof 1 to 6% may be adequate. Again, at a MW of 600,000 a concentration of1 to 4% is satisfactory, while at a MW of 4 × 10⁶ a concentration as lowas 0.2% may give good fibreization.

The concentration of the fibreizable polymer will depend upon the amountrequired to provide adequate fibre properties, and will be influencedalso by the need to produce a liquid of appropriate viscosity and speedof fibre hardening. Thus in the case of a dispersion we may use aconcentration within the range 25% w/w to saturation, (in the case of adispersion, `saturation` means the maximum concentration which may beincluded without destroying the useful spinnability of the liquid)preferably 40 to 70% and more preferably 50 to 60%, and in the case of asolution we may use a concentration within the range 10 to 60% w/w,preferably 20 to 35% w/w.

It will be appreciated that the concentration of the components musteach be adjusted to take account of the presence and concentration ofany other and their relative effects upon viscosity, etc.

The spinning material should have some electrical conductivity, althoughthis may vary between quite wide limits; for example we prefer to employsolutions having conductivity within the range 1 × 10⁻⁶ to 5 × 10⁻² mhoscm⁻¹.

Any convenient method may be employed to bring the spinning materialinto the electrostatic field, for example we have supplied the spinningliquid to an appropriate position in the electrostatic field by feedingit to a nozzle from which it is drawn by the field, whereuponfibreization occurs. Any suitable apparatus can be employed for thispurpose; thus we have fed the spinning material from a syringe reservoirto the tip of an earthed syringe needle, the tip being located at anappropriate distance from an electrostatically charged surface. Uponleaving the needle the material forms fibre between the needle tip andthe charged surface.

Droplets of the spinning liquid may be introduced into the field inother ways, which will be apparent to the skilled man, the onlyrequirement being that they can be held within the field at a distancefrom the electrostatically charged surface such that fibreizationoccurs. For example they could be carried into the field on, say, acontinuous carrier, e.g. a metal wire.

It will be appreciated that where the liquid is fed into the fieldthrough a nozzle, several nozzles may be used to increase the rate offibre production. Alternative means of bringing the fibreizable liquidinto the charge field may be employed, for example a perforated plate(the perforations being fed with fibreizable liquid from a manifold) maybe employed.

In one embodiment the surface to which the fibres are drawn is acontinuous surface, as of a drum, over which passes a belt which may bewithdrawn from the region of charge, carrying with it the fibres whichhave been formed and which have become attached thereto. Such anarrangement is shown in the attached drawings in which FIG. 1 is adiagrammatic side view of apparatus for the continuous production offibres. In FIG. 1, 1 is an earthed metal syringe needle supplied from areservoir with spinning material at a rate related to the rate of fibresproduction. Belt 2 is of gauze driven by a driving roller 3 and an idlerroller 4 to which is fed an electrostatic charge from a generator 5 (inthe apparatus illustrated a Van de Graaff machine).

Removal of the fibre mat 6 from belt 2 is by any convenient means, forexample by suction or by air jet, or it may be removed by juxtapositionof a second belt, or a second roller. Preferably it is cut and liftedoff. In the Figure the mat is shown being picked up by a roller 7rotating against the belt.

The optimum distance of the nozzle from the charged surface isdetermined quite simply by trial and error. We have found, for example,that using a potential of the order of 20 Kv a distance of 5-35 cm issuitable, but as the charge, nozzle dimensions, liquid flow rate,charged surface area etc. are varied so the optimum distance may vary,and it is most conveniently determined as described.

Alternative methods of fibre collection which may be employed includethe use of a large rotating cylindrical collecting surface substantiallyas described, the fibres being collected from another point on thesurface by a non-electrically conducting pick-up means instead of beingcarried away on the belt. In a further embodiment the electrostaticallycharged surface may be the sides of a rotating tube, the tube beingdisposed coaxially with the nozzle and at an appropriate distance fromit. Alternatively deposition of fibres and the formation of a tube mayoccur on a cylindrical former. The former may be made from any of avariety of materials. A metallic former is preferred and aluminium isparticularly preferred. The tube may be removed from the former by avariety of methods. In particular it may be mentioned that apolyurethane tube is preferably peeled from an aluminium former while analuminium former may be dissolved in sodium hydroxide solution to obtaina PTFE tube. To facilitate peeling the polyurethane tube from thealuminium former, the latter may be conveniently covered with a layer offlexible polyurethane foam.

The electrostatic potential employed will usually be within the range 5to 1000 Kv, conveniently 10-100 Kv and preferably 10-50 Kv. Anyappropriate method of producing the desired potential may be employed.Thus, we illustrate the use of a conventional van de Graaff machine inFIG. 1 but other commercially available and more convenient devices areknown and may be suitable.

It is, of course, important that the electrostatic charge is notconducted from the charged surface and where the charged surface iscontacted with ancillary equipment, for example a fibre collecting belt,the belt should be made of a non-conducting material (although it mustnot, of course, insulate the charged plate from the material to befibreized. We have found it convenient to use as the belt a thinTerylene (RTM) net of mesh size 3 mm wide). Obviously all supportingmeans, bearing etc. for the equipment will be insulated as appropriate.Such precautions will be obvious to the skilled man.

To allow high production rates, hardening of the fibres should occurrapidly and this is facilitated by the use of concentrated fibreizingliquids (so that the minimum liquid has to be removed), easily volatilesolvents (for example the liquid may be wholly or partly of low boilingorganic liquid) and relatively high temperatures in the vicinity of thefibre formation. The use of a gaseous, usually air, blast, particularlyif the gas is warm, will often accelerate hardening of the fibre.Careful direction of the air blast may also be used to cause the fibres,after detachment, to lay in a desired position or direction. However,using conditions as described in the Examples no particular precautionswere needed to ensure rapid hardening. We found that during itsformation and travel from the nozzle to the belt sufficient hardening(dehydration in the case described) occurred at ambient temperaturewithout the need for auxiliary hardening treatment.

Mats prepared according to the present invention may be between a fewmicrons and a few centimetres thick, the choice of thickness will dependon the particular application. Thus for a lining the thickness may bebetween 5 and 100 μ, preferably between 10 and 50 μ and for a wounddressing the thickness may be between 25 and 1500 μ, preferably between50 and 1000 μ.

The pore size of mats prepared according to the invention may be between0.001 and 500 μ. For linings the mat should be sufficiently porous toallow penetration of cells into it, preferably the average poredimension should be of the order of 5 to 25 μ, particularly preferablybetween 7 and 15 μ. For wound dressings the pore size will depend on thehydrophobicity of the polymer used and on the application i.e. whetheradherent or non-adherent. Typical values of average pore dimension are,for an adherent polyurethane wound dressing 50 to 100 μ and for anon-adherent polytetrafluoroethylene wound dressing 1 to 50 μ.

The as-spun mats usually have porosities in the range 55 to 95%, whichmay be reduced to as low as 1% by an appropriate compressivepost-treatment. The porosity will depend on the particular application,typical porosity values are, for a lining 75% and for an adherent wounddressing 80% and a non-adherent wound dressing 60%. By the term porositywe mean the percentage of the total volume of the mat which is freespace.

Where dispersions are employed as the spinning material, the particlesize may be between 0.01 and 1μ preferably it is between 0.1 and 0.3μ.

The high surface area of the mats according to the present inventionaffords a method of immobilising a range of active moieties so that theyare constrained to act at the site of application and do not percolatethroughout the body. Moieties which may be immobilised include enzymes,drugs and active carbon. These moieties may be added to the spinningsolutions or dispersions or the mats may subsequently be treated withthem.

While in some applications a mat of high surface area i.e. fine fibresis needed, in others a mat of high porosity is needed. Our copendingBritish Patent Application No. 41873/74 discloses methods for obtaininga desired porosity/specific area combination, namely by addition of anelectrolyte to the spinning material or by post-spinning compression ofthe mat.

PTFE and polyesters are the preferred polymers for non-wettingapplications but we do not exclude the possibility that they be used inwettable applications after incorporation of a wettable additive. Thewettable additive is preferably although not necessarily an inorganicmaterial, conveniently a refractory material, and should haveappropriate stability within the conditions of use. While the wettableadditive is preferably stable to body fluids and is not leached toorapidly, if at all, we do not exclude the possibility that reaction ordissolution may not in some instances be useful or desirable. It is alsoobviously important that the presence of the wettable additive shouldnot weaken the mat to such an extent that handling or use is made undulydifficult or that dimensional stability is affected to an undesirabledegree. The preferred additive is an inorganic oxide or hydroxide, andexamples of such materials are zirconium oxide, titanium oxide, chromicoxide, and the oxides and hydroxides of magnesium and calcium, althoughany other suitable material or mixtures of such materials may beemployed. Methods for incorporating a wettable additive into the mat aredisclosed in our copending British Patent Application No. 41873/74.

The following Examples illustrate the invention:

EXAMPLE 1

The apparatus was as shown in FIG. 1. The belt was of "Terylene" (RTM)net 15 cm wide, the nozzle diameter was 0.25 mm, located 15 cm from thesurface of the charged roll which had a diameter of 10 cm and width 16cm.

To 80 gms of an aqueous dispersion of PTFE of number average mediumparticle size 0.22 microns (Standard Specific Gravity of the polymerbeing 2.190) containing 3.6% by weight, based on the weight of thedispersion, of surfactant "Triton X100" (Rohm and Haas) and having aPTFE solids content of 60% by weight was added 20 grams of a 10% (byweight) aqueous solution of polyethylene oxide (PEO) of averagemolecular weight (MW) of 2 × 10⁵. The final composition contained 48% byweight PTFE and about 2% by weight PEO (conductivity 1.8 × 10⁻⁴ mhoscm⁻¹).

The suspension was thoroughly mixed and fed to the nozzle by an earthedsyringe injector. The electrode was charged to -20 Kv and a fine jet ofliquid was drawn from the nozzle and collected on the receiving surface.The fibers so collected were found to be dry and of even cross-section(1.0-2.0μ). The fibers were very friable and were removed carefully fromthe collector, dried at 80° C and then sintered on a bed of titaniumdioxide at 380° C for 15 minutes. After this treatment the mat, 200 μthick, was found to have retained its fibrous structure having fibersbetween 1 and 2 μ diameter and was quite strong.

The contact angle of the mat, measured by a modified method of Owens andWendt, (Journal of Applied Polymer Science 1969 13 pp 1741-1747) was137° and in the hyrostatic head test (BS 2823) a pressure of 50 cm ofwater failed to penetrate the mat.

A disc of the mat (1.3 cm diameter) was applied to the surface of awound on a rabbit involving loss of an area of full thickness skin. Therate of reepithelialisation was slightly better than that of acomparable open wound. No seepage of body fluid into the mat wasobserved.

The preparation was repeated to give an as-spun mat 800 μ thick, havinga porosity of 83% and a pore size distribution shown in Table 1. The matwas compressed to a thickness of 300 μ for 3 min at 100° C and 400 psiand then heated at 380° C for 15 minutes. The resulting mat was 400 μthick and had a porosity of 59% and a pore size distribution as shown inTable 2.

                  Table 1                                                         ______________________________________                                        Pore Size Distribution of As-Spun Mat                                                          % of Pores                                                   Pore Diameter (μ)                                                                           with Smaller Diameter                                        ______________________________________                                        100              100                                                          60               80                                                           8                50                                                           2                30                                                           ______________________________________                                    

                  Table 2                                                         ______________________________________                                        Pore Size Distribution of Treated Mat                                                          % of Pores                                                   Pore Diameter (μ)                                                                           with Smaller Diameter                                        ______________________________________                                        100              100                                                          15               85                                                           2.5              75                                                           2                67                                                           1.2              50                                                           ______________________________________                                    

EXAMPLE 2

Example 1 was repeatedexcept that 1 gm of potassium chloride was addedto the spinning composition to give a conductivity of 1.2 × 10⁻² mhoscm⁻¹. The resulting fibres, after sintering, had diameters of 0.5-1.4.

EXAMPLE 3

Example 1 was repeated except that the polyethylene oxide had an averagemolecular weight of 2 × 10⁵. The resulting fibres, after sintering, haddiameters of 0.9-1.6 μ and the mat was 50 μ thick.

The contact angle of the mat, measured as in Example 1, was 123° and themat supported a 16.5 cm column of water.

Example 4

Example 1 was repeated except that the collecting surface was a metalgauze as shown in FIG. 2, upon which the fibre mat was supported duringthe subsequent sintering process.

EXAMPLE 5

The process of Example 1 was repeated using a spinning solutioncomprising a 25% solution of a polyurethane ("Daltoflex" 330S), thedimethyl formamide/methyl ethyl ketone (conductivity 1 × 10⁻⁶ mhos cm⁻¹)the collecting surface being a metal tube (10) having a sleeve (11) offlexible open-cell polyurethane foam (see FIG. 3), the tube beingrotated at 100 rpm.

The polyurethane fibres formed had an average diameter of 2-4 microns,and were collected in the form of a tube which after completion ofspinning to give a layer about 2 mm thick could be peeled from the foam.

EXAMPLE 6

The process of Example 5 was repeated except that the product wascollected as a flat mat 75μ thick.

The contact angle of the mat, measured as in Example 1, was 73° and themat supported a 1.5 cm column of water.

A portion of the mat was tested for efficiency as in Example 1. Thehealing wound appeared neat and tidy with the absence of any grosstexture.

EXAMPLE 7

The process of Example 1 was repeated using an aluminium tubularcollector having walls 0.5 mm thick, the PTFE being collected directlyonto the metal. A first layer of PTFE was deposited on the collector, aclose helix of Nicrome wire (0.2 mm diameter) was then applied over thePTFE, followed by another layer of PTFE fibers, and the entire compositetube, on the collector, sintered. The tube was then removed from thealuminium collector by dissolving the latter in concentrated sodiumhydroxide solution.

EXAMPLE 8

The process of Example 7 was repeated using as the helical winding glassfibre of diameter 0.02 mm. Several layers of the glass fibre wereemployed. Tubes of diameters 1 to 10 cm have been prepared by themethods described in Examples 7 and 8.

EXAMPLE 9

A lining to a former of irregular contour was obtained by employing aporous conducting former and applying suction to the surface away fromthat upon which fibres were deposited sufficient to cause the fibrousmat formed to conform to the contour of the former. Such a lining couldbe attached to, say, an artificial body component, by use of anappropriate adhesive, e.g. nylon in formic acid or polyurethane in DMF.

EXAMPLE 10

A dust mask for pollen filtration was made by preparing a pattern shapedaccurately to fit over the nose and mouth, metallising the surface ofthe pattern to make it adequately conducting and depositing upon thepattern a mat about 3 mm thick of polyurethane fibres which after dryingcould be removed from the pattern and provided an accurately contouredface mask which was both light in weight and fairly elastic.

Mats obtained generally as described in Example 1, 10 cm in diameter andhaving average fiber diameter 1-2 microns, average pore size 5 micronsand porosity about 80% have been employed as air filters and where, theproperties of the polymer are appropriate, as filters for liquids e.g.water or beer. Where the mat is of hydrophobic material, e.g. PTFE,pressure may be required to force an aqueous liquid through it. Howevera degree of hydrophobicity is desirable, for example where the mat isemployed as a diaphragm for, say blood or other liquid oxygenation.

EXAMPLE 11

Using a cylindrical stainless steel former (1.6 cm diameter) charged to20 Kv, a 10% solution of polyurethane ("Daltomold" 338E) in dimethylformamide was electrostatically spun through one needle at the rate of0.7 g of PU/hour. The tubular structure produced had a wall thickness of0.4 mm, a total pore volume of 1800 mm³ /g and a median pore radius of9.4 microns and consisted of approximately 10 micron polymer nodulescross linked together by 0.4 μ diameter fibres.

A section of this product was implanted by suturing into the descendingaorta of a pig for 10 days after which the pig was killed and the graftexamined. The gross findings showed it to be patent with no sign ofintravascular thrombosis. Histologically, there was evidence ofconnective tissue and capillary ingrowth between the fibres of theprosthesis.

EXAMPLE 12

Example 1 was repeated except that a 10% by weight solution of apolyamide (Maranyl A100) in formic acid was used as the spinningmaterial. The fibers collected were dry and had even cross-sections(0.06-0.5μ).

The preparation was repeated using a 16% by weight solution of MaranylA100 in formic. The fibers collected were dry and had even crosssections (0.70-2.8μ).

EXAMPLE 13

Example 1 was repeated except that a 12% by weight solution ofpolyacrylonitrile in dimethyl formamide was used as the spinningmaterial. The fibres collected were dry and had even cross-sections(0.8-1.4 μ).

EXAMPLE 14

Example 1 was repeated except that a 10% by weight solution of apolyacrylonitrile/vinylidene chloride copolymer (Viclan A85/02) intetrahydrofuran was used as the spinning material. The fibres collectedwere dry and of even cross-section (1.0-2.0μ).

EXAMPLE 15

A wound dressing (FIG. 4) was prepared comprising a woven textilebacking 12 having an adhesive surface layer 13, a pad of absorbentmaterial 14 covered by a mat 15 of electrostatically spun materialprepared as described in Example 1. The adhesive 13 of the backing isprotected by cover strips 16 which are removed prior to application ofthe dressing.

What we claim is:
 1. A product comprising a mat of organic fibersprepared by electrostatically spinning an organic material andcollecting the spun fibers on a suitable receiver.
 2. A productaccording to claim 1 in which the mat has an average pore size of 0.001to 500 μ.
 3. A product according to claim 1 in which the mat has aporosity between 1 and 95%.
 4. A product according to claim 1 in whichthe mat is of a high molecular weight fibre forming thermoplastic, afluorinated hydrocarbon, silicone or urea/formaldehyde.
 5. A productaccording to claim 1 in the form of a wound dressing.
 6. A wounddressing according to claim 5 in which the thickness of the mat isbetween 25 and 1500 μ.
 7. A wound dressing according to claim 6 in whichthe thickness of the mat is between 50 and 1000 μ.
 8. A wound dressingaccording to claim 5 in which the mat has an average pore size of 1 to100 μ.
 9. A wound dressing according to claim 5 in which the mat has aporosity of between 60 and 80%.
 10. A wound dressing according to claim5 comprising a mat of electrostatically spun fibres in association witha backing layer.
 11. A wound dressing according to claim 5 comprising amat of electrostatically spun fibres in association with a backinglayer, and having interposed therebetween an absorbent pad, said padhaving an area smaller than, or equal to that of the mat.
 12. A productaccording to claim 1 in which enzymes, drugs or active carbon areimmobilised on the surface of the fibres.
 13. A product according toclaim 1 in which an inorganic material is incorporated in the fibres.14. A product according to claim 13 in which the inorganic material isan electrolyte.
 15. The product according to claim 1 in which the fibresof the mat comprise polytetrafluoro ethylene.
 16. A wound dressingcomprising a flexible non-absorbent hydrophobic mat formed ofelectrostatically spun organic fibres and having a thickness of between25 and 1500 microns, an average pore size between 50 and 100 microns anda porosity of 60 to 80%, the fibres having a diameter of 0.1 to 25microns and being prepared by introducing a liquid containing afibre-forming organic material into an electric field, drawing fibresfrom the liquid to an electrode and hardening the drawn fibres, and anon-absorbent backing layer.